GW Detectors Research Articles

Negative optical inertia for enhancing the sensitivity of future gravitational-wave detectors

We consider enhancing the sensitivity of future gravitational-wave detectors by using double optical spring. When the power, detuning and bandwidth of the two carriers are chosen appropriately, the effect of the double optical spring can be described as a ``negative inertia,'' which cancels the positive inertia of the test masses and thus increases their response to gravitational waves. This allows us to surpass the free-mass standard quantum limit (SQL) over a broad frequency band, through signal amplification, rather than noise cancellation, which has been the case for all broadband SQL-beating schemes so far considered for gravitational-wave detectors. The merit of such signal amplification schemes lies in the fact that they are less susceptible to optical losses than noise-cancellation schemes. We show that it is feasible to demonstrate such an effect with the Gingin High Optical Power Test Facility, and it can eventually be implemented in future advanced GW detectors.

Gravitational-Wave Constraints on the Abundance of Primordial Black Holes

We thank T. Suyama for helpful comments. Top: The energy density of the induced GWs for the power spectrum for a peak width, Δ = 0.0, 1.0 × 10−3, 1.0 × 10−1, 1.0. Bottom: Energy density of scalar-induced GWs associated with PBH formation together with current pulsar constraint (thick solid line segment) and sensitivity of various GW detectors (convex curves). Solid wedged lines indicate the energy density with the parameters (ΩPBHh2, MPBH) = (10−5, 102M⊙) (left), (10−1, 1020g) (right) for sufficiently small Δ (thick lines) and Δ = 1.0 (thin lines).

Quantum-state preparation and macroscopic entanglement in gravitational-wave detectors

Long-baseline laser-interferometer gravitational-wave GW detectors are operating at a factor of 10 in amplitude above the standard quantum limit SQL within a broad frequency band in the sense that f f. Such a low-noise budget has already allowed the creation of a controlled 2.7 kg macroscopic oscillator with an effective eigenfrequency of 150 Hz and an occupation number of 200. This result, along with the prospect for further improvements, heralds the possibility of experimentally probing macroscopic quantum mechanics MQM—quantum mechanical behavior of objects in the realm of everyday experience—using GW detectors. In this paper, we provide the mathematical foundation for the first step of a MQM experiment: the preparation of a macroscopic test mass into a nearly minimum-Heisenberg-limited Gaussian quantum state, which is possible if the interferometer’s classical noise beats the SQL in a broad frequency band. Our formalism, based on Wiener filtering, allows a straightforward conversion from the noise budget of a laser interferometer, in terms of noise spectra, into the strategy for quantum-state preparation and the quality of the prepared state. Using this formalism, we consider how Gaussian entanglement can be built among two macroscopic test masses and the performance of the planned Advanced LIGO interferometers in quantum-state preparation.

COSMIC POLARIZATION ROTATION, COSMOLOGICAL MODELS, AND THE DETECTABILITY OF PRIMORDIAL GRAVITATIONAL WAVES

CMB (Cosmic Microwave Background) polarization observations test many aspects of cosmological models. Effective pseudoscalar-photon interaction(s) would induce a rotation of linear polarization of electromagnetic wave propagating with cosmological distance in various cosmological models. CMB polarization observations are superb tests of these models and have the potential to discover new fundamental physics. Pseudoscalar-photon interaction is proportional to the gradient of the pseudoscalar field. From phenomenological point of view, this gradient could be neutrino number asymmetry, other density current, or a constant vector. In these situations, Lorentz invariance or CPT may effectively be violated. In this paper, we review these results and anticipate what more precise observations can tell us about fundamental physics, inflation, etc. Better accuracy in CMB polarization observation is expected from PLANCK mission to be launched this year. Dedicated CMB polarization observers like B-Pol mission, CMBpol mission and LiteBIRD mission would probe this fundamental issue more deeply in the future. With these sensitivities, cosmic polarization rotations from effective pseudoscalar-photon interaction, Faraday polarization rotations from primordial and large-scale magnetic field, and tensor modes effects would have chances to be detected and distinguished. The subtracted tensor-mode effects are likely due to primordial gravitational waves. We discuss the direct detectability of these primordial gravitational waves using space GW detectors.

Kg-mass prototype demonstrator for dual gravitational wave detector: Optomechanical excitation and cooling

The next generation of gravitational wave (gw) detectors is expected to fully enter into the quantum regime of force and displacement detection. With this aim, it is important to scale up the experiments on opto-mechanical effects from the microscopic regime to large mass systems and test the schemes that should be applied to reach the quantum regime of detection. In this work we present the experimental characterization of a prototype of massive gw detector, composed of two oscillators with a mass of the order of the kg, whose distance is read by a high finesse optical cavity. The mechanical response function is measured by exciting the oscillators through modulated radiation pressure. We demonstrate two effects crucial for the next generation of massive, cryogenic gw detectors (DUAL detectors): (a) the reduction of the contribution of 'local' susceptibility thanks to an average over a large interrogation area. Such effect is measured on the photo-thermal response thanks to the first implementation of a folded-Fabry-Perot cavity; (b) the 'backaction reduction' due to negative interference between acoustic modes. Moreover, we obtain the active cooling of an oscillation mode through radiation pressure, on the described mechanical device which is several orders of magnitude heavier than previously demonstrated radiation-pressure cooled systems.

Application of fast-light in gravitational wave detection with interferometers and resonators

In this paper, we study several designs for interferometric gravitational wave detectors, and the potential for enhancing their performance with a fast-light medium. First, we explore the effect of such a medium on designs similar to those already planned for Advanced LIGO. Then we review the zero-area Sagnac interferometer for GW detection, comparing its properties against the more conventional GW detector based on a Michelson interferometer. We next describe a modified version of such a detector where the Sagnac interferometer is replaced by a zero-area Sagnac ring resonator fed by an external laser. We then consider a GW detector based on an active, zero-area Sagnac ring resonator, where a gain medium is present inside the cavity. Finally, we show that if a medium with negative dispersion, which yields the fast-light effect, is also present inside this detector, then its sensitivity to GW strain is enhanced by the inverse of the group index of the dispersive medium. We describe conditions under which this enhancement factor could be as large as 105.

Matched filter for multitransducer resonant gravitational wave antennas

We analyze two kinds of matched filters for data output of a spherical resonant GW detector. In order to filter the data of a real sphere, a strategy is proposed, firstly using an omnidirectional inline filter, which is supposed to select periodograms with excitations, secondly by performing a directional filter on such selected periodograms, finding the wave arrival time, direction and polarization. We point out that, as the analytical simplifications occurring in the ideal 6 transducers TIGA sphere do not hold for a real sphere, using a 5 transducers configuration could be a more convenient choice.

Search for gravitational wave bursts by wavelet packet decomposition: The detection algorithm

We present a novel method based on wavelet packet transformation for detection of gravitational wave (gw) bursts embedded in additive Gaussian noise. The method exploits a wavelet packet decomposition of observed data and performs detection of bursts at multiple time-frequency resolutions by the extreme value statistics. We discuss the performances of detection algorithms (efficiency and robustness) in the general framework of hypothesis testing. In particular, we compare the performances of wavelet packet (WP), matched filter (MF), and power filter (PF) algorithms by means of a complete Monte Carlo simulation of the output of a gw detector, with the detection efficiencies of MF and PF playing the role of upper and lower bounds, respectively. Moreover, the performances of impulsive filter (IF) algorithm, widely used in the data analysis of resonant gw detectors, have been investigated. Results we get by injecting chirplet signals confirm the expected performances in terms of efficiency and robustness. To illustrate the application of the new method to real data, we analyzed a few data sets of the resonant gw detector AURIGA.

Wideband ‘‘bar’’ detectors and kHz gw sources: AURIGA on the Dec04 X-rays giant flare and beyond with DUAL

We discuss the interest of acoustic gw detectors for sources in the kHz range. As an example we summarize the recent upper limits in gw emission, posed by AURIGA on the Dec 2004 X-rays giant flare and see how they would improve with upgraded AURIGA in the near future. In the 2013 timeframe, a fully wideband DUAL acoustic gw detector would uniquely cover the kHz band, giving ‘‘assured’’ and, in conjunction with advanced interferometers, ‘‘confident’’ detections of the whole dynamics of coalescence of compact binaries.

Wideband mechanical amplifiers for ``dual'' gw detectors

We report on the status of the R & D towards a dual detector. We discuss the experimental advances for a broadband mechanical amplifier equipped with a Fabry-Perot cavity and for an increase of the breakdown voltage in capacitive transducers.

AIGO High Performance Compact Vibration Isolation System

We present a completed prototype of the AIGO seismic vibration isolation system. The design has been developed to satisfy the isolation requirements for the next generation of interferometric GW detectors. The system relies on passive isolation and includes multiple Ultra Low Frequency (ULF) stages to achieve minimal low frequency residual motion. Two complete isolators are being installed at the high power test facility located in Gingin, Western Australia. The performance of individual mechanical stages is continually being tested and improved. Currently it is expected that residual motion close to 1 nm at 0.3 Hz will be achieved.

Linear projection of technical noise for interferometric gravitational-wave detectors

An international network of interferometric gravitational-wave detectors is now in operation, and has entered a period of intense commissioning focused on bringing the instruments to their theoretical sensitivity limits. To expedite this process, noise analysis techniques have been developed by the groups associated with each instrument. We present methods of noise analysis that were developed and utilized for the commissioning of the GEO 600 detector. The focal point of this paper is a technique called noise projection that is used to determine the levels of contribution of various noise sources to the detector output. Example applications of this method to control loops typical of those employed in an interferometric GW detector are presented. Possible extensions of noise projections, including technical noise subtraction and gravitational-wave vetoes are also discussed.

Some considerations about future interferometric GW detectors

The two most important issues in designing future gravitational wave interferometric detectors are, in our opinion, thermal and optical noise. In this paper we discuss some approaches for reducing these noises. About thermal noise we show a possible cryogenic solution. About optical noise (namely, the combined effect of shot noise and radiation pressure noise) we show that it is possible to apply a recently proposed reduction strategy based on a ‘quantum feedback’ (Courty et al 2003 Phys. Rev. Lett. 90 083601; Courty et al 2003 Preprint gr-qc/0301068) to a full Fabry–Perot cavity. This can be seen as a further step towards the practical realization of a detector which is able to evade the standard quantum limit.

Influence of grooves and defects on the sapphire test mass Q-factor

Laser interferometer GW detectors require the test mass to have very high Q-factor normal modes to minimize thermal noise. There have been proposed two types of test mass suspension for advanced interferometers: silica or sapphire wires and membrane flexure suspensions. However, the grooves in the test mass which are required for flexure suspensions could degrade the resonance Q-factor of the test mass. We present Q-factor measurements of a suspended 4.5 kg sapphire test mass. Issues associated with the fabrication and implementation of such suspensions are discussed. Results provide strong evidence that grooves cut in sapphire have a negligible effect on the test mass Q-factor. However, suspension losses associated with bad surface contact of the flexure element do introduce significant losses.

The AURIGA second scientific run and the dual detector of gravitational waves

We report on the present status of the AURIGA gravitational wave (gw) detector, which is entering its second scientific run: the major changes which it has been subjected to are described along with the expected performance. We also report on a novel kind of acoustic gw detector, named dual detector, which would be broadband; we illustrate the concept of the selective readout, which helps in rejecting thermal and back-action contribution from non-gw sensitive acoustic modes. A SiC dual detector, 2.8 m in diameter and equipped with a wide area selective readout, would reach spectral strain sensitivities below 1×10 −23 Hz −1/2 between 1.3 and 4 kHz .

Acoustic resonance widening in GW detectors: detailed modelling of the dissipation processes

Internal friction effects are responsible for line widening of the acoustic resonance frequencies in mechanical oscillators and result in damped oscillations of its eigenmodes with a decay time Q/ω. We study the solutions to the equations of motion for the case of spherical oscillators, to be used as the next generation of acoustic gravitational wave detectors, based on various different assumptions about the material's constituent equations. Quality factor dependence on mode frequency is determined in each case, and a discussion of its applicability to actual gravitational wave detectors is made on the basis of available experimental evidence.

Hollow sphere: a flexible multimode gravitational wave antenna

Hollow spheres have the same theoretical capabilities as the usual solid ones, since they share identical symmetries. The hollow sphere is, however, more flexible, as thickness is an additional parameter one can vary to approach given specifications. I will briefly discuss the more relevant properties of the hollow sphere as a GW detector (frequencies, cross-sections), and suggest some scenarios where it can generate significant astrophysical information.

A wideband and sensitive GW detector for kHz frequencies: the dual sphere

We discuss the new concept of a sensitive and wide-band detector, consisting of a solid sphere nested inside a hollow one: the dual sphere. The advantage is that it would be possible to keep both the omni-directionality and high sensitivity of the spherical geometry without giving up the wide band. In the few kHz range the dual sphere would be complementary to ‘advanced’ interferometers. We also discuss the main technological and scientific challenges that the construction of such a system poses, particularly regarding material choice, fabrication, cooling, suspension and readout.

Silica research in Glasgow

The Glasgow group is involved in the construction of the GEO600 interferometer as well as in R&D activity on technology for advanced gravitational wave detectors. GEO600 will be the first GW detector using quasi-monolithic silica suspensions in order to decrease thermal noise significantly with respect to steel wire suspensions. The results concerning GEO600 suspension mounting and performance will be shown in the first section. Section 2 is devoted to the present results from the direct measurement of thermal noise in mirrors mounted in the 10 m interferometer in Glasgow which has a sensitivity limit of 4 × 10−19 m Hz−1/2 above 1 kHz. Section 3 presents results on the measurements of coating losses. R&D activity has been carried out to understand better how thermal noise in the suspensions affects the detector sensitivity, and in section 4 a discussion on the non-linear thermoelastic effect is presented.

Parametric adaptive filtering and data validation in the bar GW detector AURIGA

We report on our experience gained in the signal processing of the resonant GW detector AURIGA. Signal amplitude and arrival time are estimated by means of a matched-adaptive Wiener filter. The detector noise, entering in the filter set-up, is modelled as a parametric ARMA process; to account for slow non-stationarity of the noise, the ARMA parameters are estimated on an hourly basis. A requirement of the set-up of an unbiased Wiener filter is the separation of time spans with ‘almost Gaussian’ noise from non-Gaussian and/or strongly non-stationary time spans. The separation algorithm consists basically of a variance estimate with the Chauvenet convergence method and a threshold on the Curtosis index. The subsequent validation of data is strictly connected with the separation procedure: in fact, by injecting a large number of artificial GW signals into the ‘almost Gaussian’ part of the AURIGA data stream, we have demonstrated that the effective probability distributions of the signal-to-noise ratio χ2 and the time of arrival are those that are expected.